1. Field of the Invention
The present invention relates to a MEMS power inductor and, more particularly, to a MEMS power inductor with magnetic laminations formed in a crack resistant high aspect ratio structure.
2. Description of the Related Art
A micro-electromechanical system (MEMS) power inductor is a semiconductor structure that is fabricated using the same types of steps that are used to fabricate conventional analog and digital CMOS circuits, (e.g., the deposition of layers of material and the selective removal of the layers of material).
FIGS. 1A-1C show views that illustrate a MEMS power inductor 100 as taught by U.S. Pat. No. 8,044,755, which issued on Oct. 15, 2011 to Peter Smeys et al. FIG. 1A shows a plan view, while FIG. 1B shows a cross-sectional view taken along line 1B-1B of FIG. 1A, and FIG. 1C shows a cross-sectional view taken along line 1C-1C of FIG. 1A.
As shown in FIGS. 1A-1C, inductor 100 is formed on the top surface of a conventionally-formed semiconductor structure 108, which includes a substrate 110 and a metal interconnect structure 112 that touches substrate 110. Substrate 110 has a number of structures, such as resistors, transistors, capacitors, diodes, and similar devices, which are formed in and on substrate 110.
Metal interconnect structure 112, in turn, is a multi-layered structure that electrically interconnects together the various devices that are formed in substrate 110 to realize an electrical circuit. Metal interconnect structure 112 has a number of layers of metal structures, including layers of metal traces, a layer of contacts that touch and lie between the conductive regions of substrate 110 and the metal traces in the lowest layer of metal structures, and layers of vias that touch and lie between vertically adjacent metal traces.
In addition, the top layer of metal structures has a number of bond pad structures 114, including bond pad structures 114A and 114B that represent the input and the output nodes of a MEMS inductor. Further, metal interconnect structure 112 has a layer of passivation material 116 that touches the bond pad structures 114, including bond pad structures 114A and 114B. Passivation layer 116 has a number of openings that expose the bond pad structures, including openings that expose the bond pad structures 114A and 114B.
As further shown in FIGS. 1A-1C, inductor 100 includes a stress relief layer 120 that lies on passivation layer 116. Stress relief layer 120, which prevents wafer bow, is able to laterally deform enough to absorb dimensional changes from the remaining materials used to form inductor 100, and thereby prevent stress from being transmitted to the underlying metal interconnect structure 112 and substrate 110. Stress relief layer 120 has openings that expose the bond pad structures 114A and 114B.
As additionally shown in FIGS. 1A-1C, inductor 100 includes a non-conducting lower structure 124 that touches stress relief layer 120, and a number of magnetic lower laminations 126 that are formed in lower structure 124. Lower structure 124 has openings that expose the bond pad structures 114A and 114B.
Further, inductor 100 includes a magnetic gap dielectric layer 130 that is formed on lower structure 124 and the magnetic lower laminations 126. Magnetic gap dielectric layer 130 has openings that expose the bond pad structures 114A and 114B. Passivation layer 116, lower structure 124, and magnetic gap dielectric layer 130 electrically isolate each of the magnetic lower laminations 126.
Inductor 100 further includes a copper structure that includes a (square) circular copper trace 132 that touches magnetic gap dielectric layer 130, and a pair of copper plugs 134 that extend down to touch the bond pad structures 114A and 114B. Copper trace 132, which lies directly over each of the magnetic lower laminations 126, is illustrated in FIGS. 1A-1C with a single loop, although copper trace 132 can alternately be formed to have multiple loops.
As further shown in FIGS. 1A-1C, inductor 100 includes a non-conducting base structure 140 that is formed on magnetic gap dielectric layer 130 and copper trace 132, and a non-conducting cap structure 142 that is formed on base structure 140. In addition, inductor 100 includes a number of magnetic upper laminations 144 that are formed in cap structure 142.
As shown in FIGS. 1A-1C, each magnetic upper lamination 144, which lies directly over copper trace 132, has via sections that extend down so that each magnetic upper lamination 144 lies along three cross-sectional sides of circular copper trace 132, while a corresponding magnetic lower lamination 126 extends along the fourth cross-sectional side of circular copper trace 132.
Inductor 100 also includes a passivation layer 146 that touches and lies over cap structure 142 and the magnetic upper laminations 144. Base structure 140, cap structure 142, and passivation layer 146 electrically isolate each of the magnetic upper laminations 144. In addition, openings 148 can be formed in base structure 140, cap structure 142, and passivation layer 146 to expose the copper plugs 134.
In operation, a current can flow into inductor 100 through bond pad structure 114A and out through bond pad structure 114B. A current can also flow in the opposite direction, flowing into inductor 100 through bond pad structure 114B and out through bond pad structure 114A. A current flowing through an inductor generates a magnetic field which produces a magnetic flux density. The magnetic flux density, in turn, is a measure of the total magnetic effect that is produced by the current flowing through the inductor.
Lower structure 124, magnetic gap dielectric layer 130, base structure 140, and cap structure 142 can each be formed with SU-8, which is a photo-patternable epoxy resin film. One advantage of SU-8 is that SU-8 has a high aspect ratio, which means that SU-8 can be formed to have openings that are much deeper than the widths of the openings. Magnetic laminations formed in deep narrow openings are thin and thereby minimize eddy currents. As a result, high aspect ratio openings are necessary to minimize eddy currents.
One problem with SU-8 is that SU-8 is relatively brittle, and more likely to crack than other photo-patternable materials, thereby potentially exposing inductor 100 to environmental contaminants. However, photo-patternable materials which have less stress and are more crack resistant have unacceptably low aspect ratios, which means that the openings are shallower and/or wider. Magnetic laminations formed in shallower and/or wider openings are thicker and thereby have unacceptably large eddy currents. Thus, there is a need to replace SU-8 with a photo-patternable material that has both a high aspect ratio and high crack resistance.